The invention relates to gas turbine casings, and more particularly to fan retention casings for gas turbine aeroengines.
In a gas turbine aeroengine, the fan casing performs several functions. It defines an air inlet flow section for the engine, it supports an abradable material facing the tips of the fan blades, it optionally supports a structure for absorbing sound waves to perform acoustic treatment at the inlet to the engine, and it incorporates or supports a retention shield. The retention shield constitutes a trap for debris that retains any debris such as ingested foreign bodies or fragments of damaged blades projected by centrifuging, so as to prevent them from passing through the casing and reaching other portions of the aircraft.
It is common practice for a fan retention casing to be constituted by a relatively thin metal wall defining the inlet flow section and supporting the abradable material and the acoustic treatment, if any, together with a shield structure that is fastened onto the outside of said wall, in register with the fan. Such a shield structure can be formed by layers of fiber texture. By way of example, reference can be made to the following documents: U.S. Pat. Nos. 4,699,567; 4,902,201; and 5,437,538.
Proposals are made in document EP 1 674 244 to make a fan retention casing of constant thickness out of fiber/resin type composite material by making a fiber preform, impregnating it with a resin, and molding the resin-impregnated preform so as to obtain a desired shape directly. The preform is made by a kind of three-axis weaving.
Document US 2006/0093847 also relates to making a fan retention casing, but by forming extra thicknesses by means of layers of metal honeycomb cores interleaved between layers of fiber/resin composite material assembled thereto by adhesive, for example.
Document EP 1 674 671 also proposes making a fan retention casing out of a composite material of varying thickness, with its thickness being greater in register with the fan. Core fiber layers are superposed that are made of circumferentially-aligned braids. Other fiber layers are added that are made of spiral-wound braided fibers. The fiber layers are connected together by a thermosetting resin. In the event of an impact, kinetic energy is dissipated by delamination, i.e. by the fiber layers separating, by the resin cracking, and ultimately by the fibers breaking.
Documents US 2005/084377 and US 2006/257260 disclose a method enabling a fiber preform to be made for a composite material fan blade, the fiber preform being obtained directly as a single part by three-dimensional weaving with thickness, width, and weave varying during the weaving.